The present invention relates to a method of predicting the approaching stall of an aircraft wing.
An artificial stall warning device is almost universally fitted to transport aircraft. The role of the equipment is to augment, or substitute for, the natural stall symptoms, which may vary according to the aircaft configuration, weight, attitude, and the manoeuvre being performed. Surprisingly, even complex aircraft often have to rely on basic stall detection devices. These fall generally into two groups:
1. Those actuated by a hinged vane mounted on the leading edge of the wing, sensitive to the position of the stagnation point of the airflow. PA1 2. The more sophisticated angle-of-attack systems.
The wing leading edge device senses the movement of the stagnation point as it transitions from above the vane to below it, as the stall is approached. Inherent in such a system are several disadvantages:
1. The vane is very prone to interference from gusts, and, in addition, is badly affected by transient g-loadings that arise in turbulence. low speeds (where a given gust velocity is a greater percentage of the aircraft speed, and the inertia of the machine is low, which compounds the g-loading problem). Because of this, the system is most likely to be ignored just when it is most needed during take-off and landing. At best, the spurious warnings are distracting at a critical time.
2. A problem arises because of the limited number of vanes that are fitted to the aerofoil. Having just one or two sensors is undesirable because only a small part of the airflow is sampled. In fact, a second vane is often required because a single unit cannot cope with changes of aircraft configuration (the lowering of flap, for example). This serves to highlight the inherent inflexibility of such a system.
3. Another serious problem relates to flight through icing conditions, which may alter the performance of the device in several ways, even if the vane itself is heated. When an aerofoil ices up, its profile is altered. The result may be that the stalling incidence (angle-of-attack) is reduced. The vane is then referenced to the wrong stalling angle, so that no warning may be provided even though the wing may have stalled. A similar situation arises for different reasons due to the locally disturbed flow at the wing leading edge. Flow distortion influences the position of the stagnation point so that the system again becomes inaccurate.
4. There is a further problem during take-off. While the aircraft is on the ground, the incidence is basically dictated by the geometry of the aircraft. Until the nose of the aircraft is raised (termed "rotation"), an angle-of-attack sensor cannot operate. If a wing is iced up, take-off may be possible due to the ground effect, but no stall warning would be given until too late--it is procedurally incorrect to abort once rotation has started.
These last three problems arise because the sensor is calibrated for only one given situation. The system response does not adapt to altering circumstances, and indeed it cannot. It is not what might be called an `intrinsic` stall warning device. The position of the stagnation point may or may not give valid information about the stall.
Even the angle-of-attack sensor is open to similar criticism. Although local flow disruption influences the system less, it is nevertheless unable to sense a change of situation and respond accordingly.
The present invention seeks to provide a system which is dependent on an inherent characteristic of the airflow as a stall is approached.